1. Tính nhanh
a) \(\frac{1995.1997-1}{1996.1995+199_{ }}\) b) \(\frac{2}{3.5}\) + \(\frac{2}{5.7}\) + \(\frac{2}{7.9}\) + \(\frac{2}{9.11}\) + ... + \(\frac{2}{13.15}\)
c ) \(\frac{2}{1.2}\) + \(\frac{2}{2.3}\) + \(\frac{2}{3.4}\) + ... + \(\frac{2}{8.9}\) + \(\frac{2}{9.10}\) d) \(\frac{1}{3}\) + \(\frac{1}{9}\) + \(\frac{1}{27}\) + ... + \(\frac{1}{729}\)